The Smiler rollercoaster at Alton Towers Resort where two carriages crashed on 2 June Getty

Human error may have caused the Smiler crash at Alton Towers, which forced two women to have leg amputations, according to reports.

Student Leah Washington, 18, had her left leg amputated, while Vicky Balch, 20, a dancer, had her right leg amputated below the knee after the carriage they were onboard crashed into an empty vessel already on the Smiler track.

Joe Pugh, Washington’s boyfriend, also 18, endured both of his knees being smashed during the collision. Daniel Thorpe, 27, meanwhile, suffered a broken leg and a punctured lung.

John Pugh, 18, and Leah Washington, 17, both from Barnsley, were on the Smiler ride on Tuesday 2 June when their carriage collided with a stationary car on the track (Image: Facebook)

Before the incident, an engineer is alleged to have overridden an automatic safety lock on the ride, allowing a ride operator to send a carriage full of passengers onto the track despite, the Mail on Sunday reported.

The vital locking mechanism on the ride is designed to trigger braking devices, meaning two carriages should not be on the same section of the track at one time.

It is understood the members of staff who are being facing a criminal investigation by the Government's health and safety regulator are still working at the park, but in different roles.

Washington recently spoke out about the devastating incident for the first time, and said it has led her to take a “different path and a different life” and made her feel “nervous for the future”.

"My life is on hold while my friends are moving forward, having their own cars and leaving home," she told the Mail on Sunday earlier in August.

The entrance to Alton Towers, where the Smiler is based (Image: Getty)